This application is based on Patent Application No. 2001-187109 filed Jun. 20, 2001 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a calibration apparatus, an ink jet printing apparatus, a calibration method, and a medium on which a test image for calibration is printed, which all serve for a calibration which makes printing characteristics of a printing apparatus, such as a printer, to be constant, and in particular, to a test image used for the calibration that makes it possible to reduce an effect of variation in printing characteristics on calibration when printing a test pattern.
2. Description of the Related Art
Color input or output devices including input devices such as scanners and digital cameras and output devices such as monitors and printers have expressible specific color spaces, respectively. Thus, essentially, colors displayed on the monitor appear different when output from a printer. To eliminate this difference, in a system or environment using the above input and output devices, color matching between these devices is carried out by using profiles, i.e., data representative of color transformation characteristics for the respective devices.
For example, an output profile for a printer is generated as follows during a printer calibration process. First, on the basis of predetermined patch data consisting of signal values for R (red), G (green) and B (blue), or C (cyan), M (magenta), Y (yellow) and K (black), i.e., color signals for a color space dependent on the printer, the printer, for which the profile is to be generated, outputs a patch pattern. Next, the patch pattern is subjected to colorimetry using a densitometer or the like, to determine values such as XYZ or Lab, i.e., a color signal for a color space not dependent on the printer. Then, the relationship between the signal values for, for example, R, G, and B for the color space dependent on the printer, and the signal values for, for example, X, Y, and Z for the color space not dependent on the printer, is found. The thus found relationship between the RGB values and the XYZ values is used to determine a masking coefficient on the basis of an interaction method or a mapping from the RGB values to the XYZ values. Then the transformation relationship from the XYZ values to the RGB values, i.e., the reverse of the above transformation relationship, is determined as color modification data.
The profile thus obtained is used, for example, for an image processing executed when image data on the monitor is output by the printer. Then, the colors displayed on the monitor appear substantially the same as what is output by the printer.
In the above-described profile generating process, in which the transformation relationship from the RGB or CMYK signal values to the XYZ or Lab values is determined, as described above, generally, color patches are output and their density measured using a colorimeter or a densitometer so as to generate a correspondence table for the RGB or CMYK values and the XYZ or Lab values on the basis of the results of the measurements.
A printing apparatus such as a printer for which the above-described profile is generated may print an image with a different density depending on a printing position on a sheet even when the image is printed on the same sheet. For example, in a case of an ink jet printer, as a printing head that ejects ink to perform an ejection operation, generally, the temperature of the head increases. As a result, even if signals with the same value are input, the resulting amount of ink ejected may increase consistently with temperature. Consequently, as printing operations are sequentially performed on the sheet, the temperature of the printing head may vary, thereby varying the density depending on the printing position on the sheet. This also applies to the printing of the above-described patch pattern.
To verify such a variation in density, FIG. 1 schematically shows the distribution of the measured optical densities of a plurality of patches printed on the same sheet, which are gray patches of the same value for the R, G, and B signals, for example, R=G=B=192 as shown in FIG. 3, and are arranged in length and breadth directions to form a matrix pattern. In FIG. 1, for simplification of description and illustration, the measured densities of these patches are continuously expressed in the sheet though the patches are separated from one another. Further, the density of the patch is expressed on the basis of the density of lines in such a manner that the density of the patch increases in proportion to the density of the lines. Furthermore, FIG. 3, referenced above for the signal values, shows the contents of a distribution table (color separation table) that allows the R, G, and B signal values to be transformed into signals corresponding to the respective color inks actually used by the printer. The example shown in FIG. 3 relates to a printer using cyan (C), magenta (M), yellow (Y), and black (K) inks, as well as light cyan (lc) and light magenta (lm) inks, which have lower dye concentration than the above group of inks. Further, FIG. 3 shows a part of the table, which allows the R, G, and B signal values to be transformed into signal values for the corresponding inks, i.e., the figure shows the case in which R=G=B=192. Besides, according to this table, when R, G, B signals have values R=G=B=192 as referenced above, the yellow Y, light cyan lc, and light magenta lm inks are used for printing.
As shown in FIG. 1, the printing head performs a scanning operation in a main-scanning direction as shown by the arrow in the figure. During the scanning operation, ink is ejected through ink ejection openings of the printing head to carry out printing. Then, while the printing head is moving in the direction opposite to the main-scanning direction, shown by the arrow, the sheet is fed in a sub-scanning direction. Printing for the entire page of the sheet is performed by repeating the scanning operation of the printing head and the sheet feeding operation.
As is apparent from this figure, during the scanning operation of the printing head, the density increases along the main-scanning direction from a printing start position and along the sub-scanning direction.
FIG. 2 shows a distribution of densities similar to that of FIG. 1, wherein signal values for the patch pattern are used to eject inks so that the amount of ink or the number of ink types landing per unit area is increased compared to the patch pattern shown in FIG. 1; for example, R=G=B=96 is used in FIG. 2. This figure indicates that the tendency described in FIG. 1 becomes more significant as the total amount of ink landing per unit area increases. Further, when the number of ink types used for printing increases, this increasing easily causes the number of times of driving to be different between respective nozzles of ink types, which communicate with respective ejection openings, and thereby an ejection amount of respective nozzles of ink types individually vary so that difference in color tones between the printing positions becomes greater. That is, a rate of variation in density on the sheet becomes greater, and therefore a difference in density between the printing positions on the sheet becomes greater.
Further, a temperature variation associated with an ejecting operation of the printing head, which may cause the density to be varied as shown in FIGS. 1 and 2, generally behaves in such a manner as to gradually approach a certain relatively high temperature. This behavior basically depends on the heat accumulation and radiation characteristics of the printing head. More specifically, as the printing position in the sheet in FIG. 1 or 2 moves rightward and downward, the temperature increases as well as a difference in temperature between printing positions becomes small.
Furthermore, of course, a variation in temperature of the printing head or the variation in density resulting therefrom occurs not only during one directional scanning shown in the above-described example but also during scanning in bi-directional printing in which printing is executed both in one direction and an opposite direction. The behavior of variations in this case is such that as the printing position on the sheet in FIG. 1 or 2 moves downward, the temperature or density increases.
The patch pattern mentioned in FIGS. 1 and 2 is of the same signal values for printing the patches. However, this pattern is used to explain the variation in density or temperature for the same signal values. Of course, for a patch pattern typically used for a calibration, a plurality of patches with different signal values are printed.
Furthermore, another factor in the density variation associated with the variation in temperature is increasing in dye concentration of ink in the nozzle in the printing head, as shown in FIG. 23.
As shown in FIG. 23, dye concentration of ink in a nozzle increases during a relatively long interval of non-printing at an ambient temperature or during an interval of non-ejection state of the printing head in a state that the temperature of the printing head becomes high after continuous printing operation, because a solvent for the dye evaporates while the dye does not evaporate. Therefore, at a beginning of printing after the relatively long interval of non-printing or at a beginning of printing after the interval of non-ejection state of the printing head in continuous printing operation, the dye concentration of ejected ink becomes high and then the printed density increases.
It is also known that another factor in the variation in printing density on the same sheet is that associated with driving of the printing head for scanning. For example, the printing head is driven as shown in FIG. 4 on a movement for scanning in the mainscanning direction.
If it is assumed that the ink is ejected at equal time intervals while the printing head is being moved, dots are densely formed in areas where the printing head is moved at lower speed for scanning, while dots are sparsely formed in areas where the printing head scans at higher speed. On the other hand, in the example of driving shown in FIG. 4, in the areas other than those in which the printing head is moved at a constant speed, i.e., in acceleration and deceleration areas, the speed itself varies. In spite of this, typically, printing is also carried out in these areas (those areas in FIG. 4 which are designated xe2x80x9careas of density fluctuation caused by fluctuated movement speed of the printing headxe2x80x9d) in order to reduce the dimension of an apparatus in the width direction of the sheet used. However, in these areas, the speed is lower than in those areas in which the speed is constant and highest. Further, in these areas, the speed varies relatively significantly. Thus, at the side ends of the sheet, corresponding to xe2x80x9carea of density fluctuation caused by fluctuated movement speed of the printing headxe2x80x9d, even if the same head driving signal is used for printing, dense dots tend to be formed to provide high density printing compared to the center of the sheet.
As described above, even with the same signal values, the printing density may vary depending on the print position on the sheet. In such a case, the measured density of a patch pattern printed for calibration does not precisely reflect the normal printing characteristics of the printer. As a result, calibration data such as the above-described RGB values (or CMYK values, or CMYK values and lclm values associated with light color inks)xe2x80x94XYZ values (or Lab values) correspondence table which is generated based on the measured density may be imprecise. Correspondingly, a printer output profile obtained on the basis of the calibration data may also be imprecise.
For example, Japanese Patent Application Laid-open No. 7-209946 (1995) discloses a known configuration that reduces a variation in measured data dependent on the print position in the sheet when a patch pattern such as the one described above is printed. That is, as shown in FIG. 5, patches are printed so as to be randomly arranged in the sheet, so that the patches present within one area of the color space (the patches of the R, G, and B values being close to each other) are positionally distributed. Accordingly, all patches of the above one area of color space are prevented from being affected by the nonuniformity of printing within the same sheet as described above. Furthermore, for a certain particular patch, a plurality of patches, which have the same color (density), are repeatedly printed, and the average of the measurements of the patches of the same color is taken as measured data for this color, thereby improving printing-measurement precision for some colors. Thus, data, on the measured density for each print position in the sheet, is obtained as one having less bias. Further, in the above publication, as shown in FIG. 5, the ends (the periphery) of the sheet are made non-printing areas, so that the area for printing the patch pattern is more toward the interior of the print sheet, thereby preventing a variation in density resulting from a variation in movement speed of the head at the ends of the sheet.
However, even though measured data obtained by randomly arranging the patches is such that all patches of colors within one area of the color space (the R, G, and B values are close to each other) are prevented from varying depending on the print position in the sheet, as described in the above publication, the measured data is likely to be data having bias about the variation in printing density caused by an increase in head temperature associated with a scanning operation of the printing head. More specifically, in the case of one-directional printing, the variation in density caused by the increase in head temperature associated with a scanning operation of the printing head generally gradually increases from a corner of the sheet (printing start position A) toward such a corner thereof (printing end position B) that these two corners are point-symmetric with respect to the center of the sheet, as shown in FIGS. 1 and 2. That is, this variation has a certain tendency. Thus, in measured data obtained from randomly arranged patches or in the mean value of measured data obtained by spatially randomly arranging some patches, this certain tendency may appear relatively markedly. That is, the randomly arranged patches are affected by the tendency of the variation in density correspondingly to the positions thereof.
Further, even if the area of non-printing is simply provided in the sheet as in the above publication, it is apparent that, though the variation in density resulting from a variation in movement speed of the printing head may be prevented at a home position side of the printing head because a serial printer has, for example, control of the movement of the printing head such that after scanning for printing in one direction a speed of the printing head is reduced at a short distance and the printing head is made to return to the home position, the above-described variation in colorimetric data attributed to the variation in the head temperature cannot be reduced.
Further, a method disclosed in the publication cannot reduce a variation in colorimetric data attributed to increasing of dye concentration in the nozzle, which occurs after an interval between continuous printing operations.
An object of the present invention is to provide a calibration apparatus, an ink jet printing apparatus, a calibration method, and a print medium having a calibration test image printed thereon, which all serve to print a patch pattern that enables measurements of patches that precisely reduce an effect of a variation in density in the patch pattern on the measurement, the variation resulting from a variation in head temperature, a variation in movement speed and a variation in dye concentration of ink in a nozzle of a printing head.
In the first aspect of the present invention, there is provided a calibration apparatus for outputting test image data to cause a printing apparatus to print a test image used for a calibration for the printing apparatus,
wherein the test image includes a measure image which is a subject of a measurement and a dummy image which is not a subject of the measurement, and the dummy image is printed at least at a part of a periphery of an area on which the measure image is printed, on a printing medium.
Here, the printing apparatus may be one that repeats scanning of a printing head across the printing medium and transporting of the printing medium at a predetermined amount in a direction different from a direction of the scanning of the printing head so as to print the test image, and the test image may include dummy images printed at both ends of a scanning range of one scanning of the printing head and the measure image printed so that the measure image is positioned between the dummy images of the respective ends.
The printing apparatus, based on the test image data, may print a pair of the test images which include the respective measure images whose print positions in the printing medium are symmetrical to each other with respect to a center of an arrangement of the measure images.
In the second aspect of the present invention, there is provided an ink jet printing apparatus which uses a printing head ejecting ink to print a test image used for a calibration,
wherein when printing the test image ink ejection is executed from the printing head on an area other than an area on which the test image is printed.
In the third aspect of the present invention, there is provided a calibration method including a process for outputting test image data to cause a printing apparatus to print a test image used for a calibration of the printing apparatus,
wherein the test image includes a measure image which is a subject of a measurement and a dummy image which is not a subject of the measurement, and the dummy image is printed at least at a part of a periphery of an area on which the measure image is printed, on a printing medium.
Here, the printing apparatus may be one that repeats scanning of a printing head across the printing medium and transporting of the printing medium at a predetermined amount in a direction different from a direction of the scanning of the printing head so as to print the test image, and the test image may include dummy images printed at both ends of a scanning range of one scanning of the printing head and the measure image printed so that the measure image is positioned between the dummy images of the respective ends.
The printing apparatus, based on the test image data, may print a pair of the test images which include the respective measure images whose print positions on the printing medium are symmetrical to each other with respect to a center of an arrangement of the measure image.
A pair of the test images may be printed which include the respective measure images whose print positions on the printing medium are symmetrical to each other with respect to a center of an arrangement of the measure image.
According to the above structure, a test image used for calibration includes measure images to be measured and dummy images that are not measured. The dummy images are printed on at least a part of a periphery of a printing medium, which is located around the area on which the measure images are printed. Accordingly, before the measure images are printed, printing of the dummy images can be performed to precisely reduce and stabilize a variation in density of patches in a patch pattern caused by a variation in a moving speed of a printing head on printing operation and a variation in dye concentration of ink in a nozzle of the printing head. More specifically, in a system including also a serial printer in which the printing head moves only across a part of a scanning area for which ejection data is present when performing a scanning operation, printing of the dummy image allows the speed change of the printing head to be shifted to a constant speed area during printing the dummy image to stabilize the speed on printing the measure images. Further, as to the variation in dye concentration of ink in the nozzle of the printing head, since ink in the nozzle is removed by printing of the dummy patch before printing the measure images, the dye concentration of ink can be made constant during printing the measure images. Thereby, a variation in printing density, which results from the variations in temperature of the printing head and in dye concentration on printing the measure images, can be reduced. Furthermore, printing of the dummy image can avoid change in a mix ratio of C, M, Y, K inks for printing the measure images, which is caused by mixing of different type inks near the ejection openings of the printing head.
According to a further preferred structure, the test image is such that the dummy images are printed at the opposite ends of a single scanning range of the printing head and the measure images printed so as to be sandwiched between the dummy images printed at the opposite ends. Accordingly, when the test image is printed by scanning the printing head, the measure images can be prevented from being printed at the opposite ends of the scanning range, where the speed may vary in connection with the scanning movement. This also hinders a variation in printing density of the measure images attributed to a variation in speed.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.